The present invention is specific to a working electrolyte for an electrolytic capacitor.
So called “wet” tantalum electrolytic capacitors were first commercially introduced in the form of sintered powder metallurgy slugs of less than theoretical density. The tantalum slugs, which have very large internal surface area, act as the positive capacitor plates in tantalum wet slug capacitors. After an anodizing step, the interstices of each anode slug are coated with an anodic oxide film which acts as the capacitor dielectric. The anodized tantalum slugs are then sealed in cans containing a highly conductive liquid electrolyte solution and having high surface area conductive linings to communicate the current to the liquid electrolyte solution.
The electrolyte solutions used in wet tantalum electrolytic capacitors have traditionally consisted of one of two basic formulations. The first formulation consists of an aqueous solution of lithium chloride. Lithium chloride solutions have the advantage of being relatively non-hazardous to personnel assembling the capacitors and of being non-corrosive to silver cases used in the fabrication of many wet tantalum capacitors, especially those capacitors used under conditions of high ripple current or reverse polarity. Silver is extremely insoluble in near neutral pH chloride solutions. Aqueous lithium.chloride solutions have the disadvantage of being limited to voltages below about 30 volts and have higher resistivity leading to higher device equivalent series resistance or ESR than the second type of electrolyte solution.
The second electrolyte formulation traditionally used in wet tantalum capacitors consists of an aqueous solution of 35-40% sulfuric acid. Aqueous sulfuric acid electrolyte solutions are both hazardous to capacitor assembly personnel and corrosive to capacitor production machinery. The resistivity of aqueous sulfuric acid is very low and, consequently, capacitors containing aqueous sulfuric acid electrolytes exhibit lower ESR than capacitors containing any other aqueous electrolyte solution. Aqueous sulfuric acid may also be used in capacitors rated up to 125 volts at 85° C. Aqueous sulfuric acid also exhibits a relatively small change in resistivity with changing temperature and so, with proper device construction, may be used in capacitors rated for service at −55° C. to 200° C. The low resistivity, wide temperature capability, and relatively high maximum operating voltage have made aqueous sulfuric acid the electrolyte of choice for the vast majority of wet tantalum capacitors manufactured currently.
Efforts have been underway for many years to extend the voltage capability of the liquid electrolyte solutions used in wet tantalum capacitors. Phosphoric acid has been added to the aqueous sulfuric acid in an attempt to raise the electrolyte sparking voltage. The addition of phosphoric acid is accompanied by an increase in resistivity and an increase in the change of resistivity with temperature. The observed increase in sparking voltage is only on the order of 15-25 volts and the negative impact upon resistivity is such that phosphoric acid electrolyte additions are seldom used in commercial devices.
The addition of approximately 1% boric acid to the conventional aqueous sulfuric acid fill or working electrolytes used in wet tantalum electrolytes has been found to increase the sparking voltage of the electrolyte by about 25 volts with little or no effect upon the electrolyte resistivity and device ESR as described in U.S. Pat. No. 4,539,146. More important than the enhanced sparking voltage observed with boric acid additions to sulfuric acid electrolytes is the reduced leakage current associated with these additions. In the case of device failure from shorting/DCL while in service, the presence of boric acid in the electrolyte tends to minimize the leakage current so as to prevent bursting of the capacitor cases from gas generation and electrolyte boiling before circuit shutdown occurs.
The substitution of one or more organic solvents for the aqueous component of the working electrolyte for wet tantalum capacitors to facilitate higher breakdown voltages is hampered by the requirement of low electrolyte resistivity made necessary by the sintered slug construction of wet tantalum capacitors. This is similar for electrolyte solutions used as fill or working electrolytes in aluminum electrolytic capacitors. Organic solvent solutions of certain salts, such as ammonium metatungstate, ammonium thiocyanate or ammonium boro-disalicylate exhibit resistivities as low as approximately 40 ohm-cm at 30° C. Ammonium nitrate solutions in dimethyl formamide may exhibit still lower resistivity. These solutions have been used successfully in low-voltage tantalum foil capacitors, however, the sparking voltages of these solutions are generally well under 100 volts in wet-slug tantalum capacitor applications.
Organic solvent-based electrolyte solutions having sparking or breakdown voltages above the 125-150 volts of aqueous sulfuric acid solutions tend to contain salts of less well ionized organic acids as the main ionogen and exhibit resistivities of hundreds of ohm-cm at 30° C. In order to be of useful service in wet tantalum capacitors, the electrolyte resistivity should be below about 50-60 ohm-cm at 30° C. and the sparking voltage or useful voltage should be significantly above 150 volts.
U.S. Pat. No. 6,219,222 describes a series of electrolytes useful as fill or working electrolytes for wet tantalum capacitors at voltages of 200+working volts (rated voltage). In order to obtain sufficiently low resistivity, the electrolytes of 6,219,222 employ a solvent system that contains about 50% or more water. The ionogens listed in U.S. Pat. No. 6,219,222 include ammonium acetate and ammonium adipate. The organic solvents listed in 6,219,222 include ethylene glycol, 4-butyrolactone, and dimethyl formamide.
Experiments conducted in our laboratory indicate that the inclusion of alkyl monocarboxylic acids in electrolyte solution formulations give rise to electrolytes which exhibit a tendency to grow anodic oxide on tantalum, at any cracks or flaws in the anodic oxide, at significantly below 100% anodizing efficiency. The reduced anodizing efficiency associated with monocarboxylic acids (particularly lower molecular weight monocarboxylic acids) may act as an additional battery drain in battery powered circuits containing wet tantalum capacitors filled with electrolyte solutions containing monocarboxylic acids.
Electrolyte solutions containing ammonium adipate have been found to be very efficient in the growth of anodic oxide on tantalum and fill or working electrolyte solutions containing ammonium adipate tend to be less wasteful of battery life in battery-powered circuits containing wet tantalum capacitors filled with ammonium adipate-containing electrolyte solutions.
Unfortunately, electrolyte solutions is containing a sufficient amount of ammonium adipate to produce desirably low resistivity at 20-40° C. tend to produce ammonium adipate precipitates when cooled below room temperature. If capacitors containing ammonium adipate electrolyte solutions are exposed to low temperatures, e.g. −25° C. or lower, the precipitation of ammonium adipate is such as to require heating to elevated temperature to restore the electrolyte solution to its original composition. The high temperature is required to re-dissolve the ammonium adipate in the solvent of the electrolyte.
What is desired is one or more dicarboxylic acid salts for the high anodizing efficiency associated with these salts in partially organic aqueous solution which have very high solubility in the electrolyte solvent to provide sufficiently low electrolyte solution resistivity. Furthermore, salts are desired which do not precipitate from solution upon cooling to at least −25° C.